Correction of linearity errors

ABSTRACT

In order to compensate for currents which are induced in a convergence yoke (16) by a deflection yoke (14), a pulse of current is injected into an amplifier (26) which receives the convergence waveform (from 28) so as to correct or compensate for the induced current. One way of generating the compensating pulse is by delaying a horizontal retrace pulse (36).

BACKGROUND OF THE INVENTION

The present invention relates to a cathode ray tube system which uses anauxiliary deflection winding for convergence correction. Such anauxiliary convergence correction winding or yoke is commonly used inprojection television systems where each one of multiple cathode raytubes generates a unique color image. When the color images areoptically combined, slight correction is necessary in order that themultiple images exactly match.

In some systems, a convergence yoke is positioned close enough to thedeflection yoke so that deflection currents are induced in theconvergence yoke. Such induced currents in the convergence yoke maycause undesirable deflection of the electron beam. The Applicant hasfound that such induced currents cause an apparent stretch of the imagenear the beginning of horizontal scan.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a cathode ray tubedisplay system in which image distortion, due to currents induced in aconvergence correction yoke by the deflection yoke, is corrected.

More specifically, the invention provides a cathode ray tube systemwhich includes a kinescope, a deflection yoke and a correction yokemounted thereon. The deflection yoke is energized by sawtooth wavecurrents to generate magnetic fields for scanning an electron beamacross the face of the kinescope. The correction yoke, when energized,generates magnetic fields which correct for beam landing errors in thekinescope. Since the correction yoke is affected by stray fields fromthe deflection yoke, in order to compensate for the effect of the strayfields on the correction yoke, a source of compensation current is fedto the correction yoke near the beginning of a deflection sawtooth wave.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of a cathode ray tube having a deflectionyoke and a convergence correction yoke.

FIG. 2 shows waveform diagrams of the current that is induced in theconvergence correction yoke by the horizontal deflection winding fordifferent values of correction yoke winding circuit impedances.

FIG. 3 is a schematic circuit diagram, in block form, of a cathode raytube display system, according to the invention, which compensates forthe currents in the correction yoke which are induced by the deflectionyoke.

FIG. 4 is a detailed circuit diagram of a correction arrangement forthree cathode ray tubes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a cathode ray tube 10 having an electron gun 12 whichgenerates an electron beam and directs it toward the face of the cathoderay tube. Deflection yoke 14 magnetically scans the beam across the faceof the tube while convergence correction yoke 16 corrects for beamlanding errors. It is to be noted that convergence correction yoke 16and deflection yoke 14 are in close proximity, so that stray fieldswhich are generated by deflection yoke 14 tend to induce currents incorrection yoke 16.

FIG. 2 shows waveforms of the current which is induced in convergencecorrection yoke 16 by the horizontal deflection field from deflectionyoke 14. Curve 18 shows the current which is induced in correction yoke16 when yoke 16 is terminated by zero impedance (a short circuit). Thiswaveform closely approximates the waveform of the horizontal deflectioncurrent. However, when convergence correction yoke 16 is terminated by a40 ohm impedance, which is the approximate value of the output impedanceof the amplifier which drives the convergence correction yoke, the shapeof the induced current changes to the shape shown by curve 20. Curve 20is a pulse occurring at the beginning of horizontal trace and having aduration of about 15% of the horizontal trace interval. The inducedcurrent reaches its baseline about 4 microseconds (μS) after the startof video.

FIG. 3 shows a schematic diagram, in block form, of a circuit forcorrecting for the current which is induced in the convergence yoke bythe horizontal deflection winding of the deflection yoke. Convergenceyoke 16 is driven by power amplifier 24 which is, in turn, driven bysignal amplifier 26. Signal amplifier 26 receives a convergencecorrection waveform from convergence generator 28 through resistor 30.Also fed to the input of the signal amplifier 26 is a pulse signal fedthrough resistor 34. The input to signal amplifier 26 is therefore thesum of the waveform generated by convergence generator 28 and the pulsewaveform which is fed through resistor 34. The pulse waveform which isfed through resistor 34 may be generated by suitably delaying a retracepulse 36 in delay 32, so that the pulse waveform input to amplifier 26occurs during the first 4 μS after the start of video in the horizontaltrace interval.

In this way, the undesired current which is induced in the convergencecorrection yoke 16 by the horizontal deflection winding is compensatedor counteracted by the pulse which is fed through resistor 34 to theinput of signal amplifier 26.

In order for pulse 36 to counteract or compensate for the current whichis induced in convergence yoke 22 by the horizontal deflection winding,the pulse must occur at the beginning of the trace interval. One way ofproducing a pulse at this time is to feed a retrace pulse 36 through adelay circuit 32 so that the retrace pulse is fed through resistor 34 tothe input of amplifier 26 at the beginning of the deflection traceinterval. However, it is apparent that other pulse sources may be used.

FIG. 4 is a circuit diagram of an arrangement for supplying correctioncurrent to each of green, red and blue display tubes of a multicoloreddisplay system. Flyback pulses 36 are fed through resistor 50 to thebase of transistor Q1. Capacitor C1 delays the application of pulses 36to the base of transistor Q1.

Transistor Q1 acts as an emitter-follower and generates pulses at itsemitter which are passed through capacitor C2 and resistor 56 to thebase of transistor Q2. Capacitor C2 may be used to control the pulsewidth. For example, a value of 15 pf will produce a narrow pulse.

Capacitor C3 acts as a feedback path from collector-to-base oftransistor Q2, in order to improve the trailing edge timing stability.

The output pulse for the red and blue kinescopes is taken from thecollector of transistor Q2 to signal amplifiers 26R and 26B. Pulses forthe green kinescope are of the opposite polarity, and are thereforetaken from the emitter of transistor Q1 to signal amplifier 26G.

I claim:
 1. A cathode ray tube display system including a kinescope, adeflection yoke and a correction yoke mounted thereon, said deflectionyoke being energized by horizontal and vertical sawtooth wave currentsto generate magnetic fields during trace intervals for scanning anelectron beam across a face of said kinescope, said correction yoke,when energized, generating magnetic fields which correct for beamlanding errors in said kinescope, said correction yoke being affected bystray fields from said deflection yoke; wherein, in order to compensatefor the effect of said stray fields on said correction yoke, said systemcomprises means for passing a pulse of compensation current through saidcorrection yoke during a trace interval near the beginning of adeflection sawtooth wave.
 2. A display system as defined by claim 1, inwhich said current pulse is generated in response to a deflectionretrace pulse.
 3. A display system as defined by claim 2, in which saiddeflection retrace pulse is coupled to said correction yoke through adelay means.
 4. A display system as defined by claim 3, in which saiddelay means comprises a transistor and a capacitor, said capacitorcoupled from base to emitter of said transistor.
 5. A display system asdefined by claim 4, in which said transistor is connected as an emitterfollower, the emitter of said transistor being coupled to saidcorrection yoke.
 6. A display system as defined by claim 4, in which theemitter of said transistor is coupled to the base of a secondtransistor, the collector of said second transistor being coupled to asecond correction yoke.
 7. A display system as defined by claim 1, inwhich said compensation current is fed to said correction yoke near thebeginning of a horizontal deflection sawtooth wave.
 8. A display systemas defined by claim 1, in which said current pulse is generated inresponse to a horizontal deflection pulse.